When I retired I was given to understand that senior officers (Sqn Ldr and above) and their equivalents in the other services could use their rank if they wished.
On retiring I reappeared as a Civil Servant: all concerned probably knew my old rank but I was Mr. Just as well as I worked for many Sqn Ldrs over the years and using my old rank would have been a tad embarrassing all round!
Now I've retired fully, at sqn reunions etc ranks reappear on the seating plan - but I couldn't fit into my No5 now!!